In some applications it is desired to determine a heading angle of an object i.e. a direction of an object with respect to a reference point. Typically the heading angle of the object should be determined with respect to a certain point in a vicinity of the object. Further, movements of the object should also be detected to perform certain actions on the basis of detected heading angle and/or movements of the object. Some of such applications relate to generating audio signals for a person so that the heading angle and movements of the person's head are taken into consideration when the audio signals are generated. The person may use a headset to listen audio signals, e.g. music, sounds of a TV program, a movie, a game, etc. Typically this kind of listening experiences take place in a room so that visual effects are shown on TV, on a computer display, on a screen, on one or more walls of a room (like in a cinema), etc. Such visual effects may include a video, a movie, a TV program, actions of an electronic game, still images, actions of a computer program, etc. It is obvious that the above mentioned examples are not restrictive but also other visual and audio information may be formed by applications in which there is a need to determine a heading angle of an object.
The determination of the heading angle can be performed in one direction or in two directions. The one-directional determination of the heading angle can be performed e.g. in a horizontal or in a vertical direction. The determination in a horizontal direction defines the horizontal angle which is usually called as azimuth. Respectively, the determination in a vertical direction defines the vertical angle which is usually called as elevation.
If the heading angle of an object, for example a person's head, is not determined, the audio signals may not sound natural when the person moves her/his head. For example, when the person is watching a movie on a screen and uses a headset to listen to the audio signals relating to the movie, the person's head should stay in a certain heading angle with respect to the screen to reproduce sounds in such a way that the person could differentiate and locate the origin of different sounds on the screen. In other words, to be able to experience the sound of the movie as natural as possible, the heading angle of the person's head should not affect to the listening experience.
Systems for tracking a heading angle of an object, such as a person's head, have been developed These solutions are usually based on transmission and reception of ultrasonic or optical signals, or they are based on inertia sensors. Also hybrid systems have been developed in which two or more different techniques are combined. In ultrasonic and optical systems signals are generated by one ore more transmitters and received by one or more receivers. For example, in the patent U.S. Pat. No. 5,107,746 three ultrasonic transmitters transmit ultrasonic signals which are received by a receiver attached with or carried by the object to be tracked. Transmitters are at fixed positions which are known to the receiver. The receiver detects differences in time-of-travel of signals from the transmitters and calculates the position of the object by using triangulation. Each transmitter transmit similar kind of pulses. Therefore, the receiver should know which the transmitter transmitted a pulse signal. In the system of U.S. Pat. No. 5,107,746 this is achieved by stagging the transmission of signals from the transmitters. Therefore, only one transmitter should transmit the signal at a time. These kind of prior art systems have certain problems. For example, more than one transmitters are needed to make the triangulation possible. Further, due to the stagging principle the transmission of the transmitters need to be synchronized with each other to avoid overlapping transmissions. The triangulation based location solution requires that the position of the transmitters is known.
In optical systems there should be a direct line-of-sight between a transmitter and a receiver or the tracking will be interrupted or becomes totally impossible. Systems based on coherent detection of a waveform (optical, acoustical, or radio signals) are sensitive to disturbances and other sources of error. The effect of errors are cumulative thus causing an increasing offset to tracking information.